Oxidation reactions of alcohols constitute one class of important reactions as methods for chemical conversion of compounds, and are frequently used in syntheses of organic compounds with high added values such as medicaments and agricultural chemicals and the like. Therefore variety of methods have been developed so far. However, many of those methods use explosive reagents, highly toxic metals and the like, or place heavy load on the environment, such as production of huge amounts of waste matters. Employment of environment-friendly and safe synthetic methods has been desired especially in the field of industrial processes, and accordingly, several methods have been developed for oxidation reaction of alcohols from the aforementioned point of view.
For example, the oxidation reaction using 2,2,6,6-tetramethylpiperidine N-oxyl (henceforth also abbreviated as “TEMPO” in the specification) is a reaction in which TEMPO serves as an organic oxidation catalyst, and an oxoammonium salt generated from nitroxy radical derived from TEMPO acts as a chemical species having an oxidation activity. This oxidation reaction can achieve oxidation at a relatively low cost, and further, the reaction does not use highly toxic transition metals, and advances even under a mild condition such as at about 0° C. to room temperature. Therefore, this reaction has recently been focused as a highly environment-friendly oxidation reaction.
As for the oxidation reaction that uses TEMPO, since from the catalytic oxidation reaction utilizing m-chloroperbenzoic acid as a bulk oxidant was reported (Non-patent document 1), methods utilizing various bulk oxidants in combination have been developed. For example, there have been reported methods of using sodium hypochlorite, which has been most widely used in the present industrial processes because of inexpensiveness and less load on the environment, or diacetoxyiodobenzene, which accepts coexistence of a wide variety of functional groups, as a bulk oxidant (Non-patent documents 2 and 3). Methods of using more environment-friendly oxygen as a bulk oxidant have also been actively studied (Non-patent documents 4 and 5).
As mentioned above, the TEMPO oxidation has given a position as a potent oxidation method in the field of organic synthetic chemistry and industrial chemical processes. However, this oxidation often has a problem of low reactivity especially in oxidation of secondary alcohols due to insufficient reactivity for a bulky substrate. In order to solve this problem, there have been proposed 1-methyl-2-azaadamantane N-oxyl (henceforth also abbreviate as “1-Me-AZADO” in the specification), and 2-azaadamantane N-oxyl (henceforth also abbreviate as “AZADO” in the specification), which are nitroxyl radicals having the azaadamantane structure, as well as 9-azabicyclo[3.3.1]nonane N-oxyl (henceforth also abbreviate as “ABNO” in the specification), which is a bicyclo type nitroxyl radical (Non-patent documents 6 and 7 and Patent documents 1 and 2). It has been confirmed that 1-Me-AZADO, AZADO, and ABNO have a catalytic activity far higher than that of TEMPO, and it has been elucidated that they have high catalytic activity not only for oxidation of primary alcohols, but also for oxidation of secondary alcohols, which hardly advances with TEMPO.
However, the synthesis of 1-Me-AZADO has a problem that it requires seven steps from a commercially available compound. Although ABNO can be synthesized by three steps from a commercially available compound, it has a lower catalytic activity compared with those of AZADO and 1-Me-AZADO when an amount added is small, and thus it has problems from environment compatibility and economical viewpoints. When air is used as a bulk oxidant, which has superior environment compatibility, each of 1-Me-AZADO, AZADO, and ABNO achieves a slow reaction rate at a catalyst amount of about 1 mol %, and thus they have a problem from a viewpoint of reaction efficiency. Although the compound represented by the following formula (I) (Nor-AZADO) is disclosed in Non-patent document 8, no use of the compound as an oxidation catalyst is suggested or taught.